iMac 5K, late 2014 model. The weirdest thing happened yesterday. This machine is usually very performant — 8GB ram, 3TB Fusion drive. I dual-boot Windows on it using Boot Camp.

As of now, I still have 980GB free — this issue therefore cannot be due to low storage.


The Mac ran perfectly fine until I used Photos.app to browse some photos (the app did a "library permissions repair"). Shortly after, I began experiencing frequent macOS user interface freezes (all apps, spinning beachball) for sometimes minutes on end.

Activity Monitor showed very low RAM pressure and barely any CPU stress at the time. Because I couldn't isolate the issue, I rebooted the Mac.

Now, even clicking the login picture hangs the computer for around 4-5 minutes. Yes, it takes that long to show the password field. After entering the password, the wallpaper appears 2 minutes in, the Dock appears in another 3 minutes, and menu bar widgets take around 2 minutes to appear each. Opening Launchpad hangs on "Loading applications..." for minutes on end, and each individual app icon loads in its own time.

More symptoms

  1. Opening anything - System Preferences, About this Mac, Spotlight, Finder takes ages, and freezes the whole interface while it's waiting. In this frozen state, only the cursor moves and nothing else responds whatsoever.
  2. If not clicking on anything, the whole interface freezes periodically for 2-3 minutes anyway.
  3. For whatever reason, opened apps work completely fine — you can drag them around, and select list items in them. It appears that doing anything that requires any I/O, like switching Finder folders or System Preference panes, however, takes forever.
  4. When I am able to open Activity Monitor, there are 0 culprits. No apps using the disk, RAM pressure is super low—although, perhaps normally, swap usage it at 0 bytes—and the CPU is virtually idle. Strange.
  5. SSHing into the Mac from another box exhibits these same symptoms, suggesting it's not loginwindow. SSH takes about a minute to log in and every command takes forever to execute.

What I tried

  • I ran Apple Hardware Diagnostics (Cmd+D on startup). No hardware issues found.
  • I ran First Aid on the APFS volume in Recovery OS (Cmd+R on startup). No major issues found, and it seemed OK.
  • I removed /var/db/.applesetupdone in Single User Mode to force macOS to create a new admin account. The admin account suffers from the same symptoms.

System info

I'm running macOS Mojave 10.14.6 Beta. Yes, I'm aware it's beta software, but it worked fine for ages until yesterday. I'm not keen on reinstalling macOS, and doing a backup is impossible as I do not own an external hard drive of the Mac's capacity. In fact, the Mac sort of was my back-up hard drive in a sense.

What can I do? Do these symptoms ring a bell? It seems like I/O to me, but I'm honestly quite lost.

  • 1
    I'd take a wild swing that the SSD has failed, leaving the HD to do all the work. What you describe sounds like my folks' iMac, which is an HD only machine, or my Mac Pro if I have to work from an HD clone of my usual setup. I'm not actually sure how you'd test for that; I don't have a Fusion drive here I can check. I'd definitely make it a priority to get it backed up, just in case.
    – Tetsujin
    Jun 4, 2019 at 8:16
  • That's an interesting guess. I did run First Aid and it said no errors were found, and Apple Diagnostics said there were no hardware errors. Do you think reinstalling macOS (through the recovery OS) would be more complicated with the Fusion + Bootcamp setup? At this point, I'm ready to do that.
    – Joseph
    Jun 4, 2019 at 8:24
  • I'd assume first aid considers the entire fusion drive as a single entity, but as I say, I don't have one I can check against. See what you get if you type diskutil cs list & diskutil list in Terminal & post those to your question.
    – Tetsujin
    Jun 4, 2019 at 8:28
  • What do you mean by "no major issues found" - can you provide the output from the First Aid check?
    – jksoegaard
    Jun 4, 2019 at 10:01
  • I hope I'm wrong, but this is "stop and back up now" territory based on my experience and your detailed analysis of things. I'll add that as an answer and hope someone has a workaround for you if you can't take my advice to triage and preserve things.
    – bmike
    Jun 4, 2019 at 11:46

1 Answer 1


Since this Mac is a backup, I would buy or find an external drive and install a clean OS there - leaving the internal drive and data untouched.

This lets you test the hardware with a clean OS with minimal risk to losing your data.

Then, since you don't have a drive big enough to back up all the data, you can wait to see if you want to throw out less important things until your data that needs to be backed up fits on the drive you have or you can fund a drive large enough to keep the data you choose to keep.

I wouldn't mess with things - slow IO and freezes out of the blue are usually two things:

  • fundamental errors / corruption to the data structures that a fsck can't or won't fix
  • hardware issues with spinning drives (SSD generally don't slow - they just fail when they can't accommodate any more bad blocks or the controller itself fails)

Both of the above, you need a backup to recover from in my experience and delaying making that backup, reduces what data you can save.

  • Thanks for your thoughts! I just reinstalled macOS, and the freezes seem largely gone (I'm now able to log in and use the computer). However, things do seem different. Random freezes still occur. I am constantly watching Console.app logs, and nothing out of the ordinary ever appears — it's like the freezes weren't there.
    – Joseph
    Jun 4, 2019 at 12:40
  • Assuming there are "fundamental errors / corruption to data structures" as you suggest, would a complete reformat of the Fusion drive fix those? I'm willing to back up my most important data and completely start over. What would be the best way to do this while getting rid of this corruption?
    – Joseph
    Jun 4, 2019 at 12:40
  • @JosephA. yes, corrupted file systems almost always can be erased and then stable in service. Disk Utility doesn’t like to put Humpty Dumpy together. It does like a clean sweep.
    – bmike
    Jun 4, 2019 at 23:58

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